Several methods exist for constructing narrow linewidth single mode semi conductor lasers. Practical devices fall into two groups, monolithic semiconductor devices, and external cavity lasers. All of these use frequency selective feedback to achieve single mode operation over a range of drive currents.
Frequency selective feedback may be provided by a periodic structure either with an external element or within the semiconductor (i.e. monolithic), which can take the form of a DBR (distributed Bragg reflector) or DFB (distributed feedback) laser. In a DBR the grating is outside the active region but in the monolithic semiconductor, whereas the grating in DFB lasers is within the active region. External cavity lasers consist of an optical gain medium that is located between, but does not occupy the full distance between, two reflectors, one reflector commonly being the back facet of the gain medium and the other an external Bragg grating which may be produced in a fibre or waveguide. In previously described devices the external grating is formed either in a single mode fibre, or a Silicon Nitride (Si.sub.3 N.sub.4) or Silica (SiO.sub.2) waveguide.
DFB lasers have a major disadvantage in that, since the output wavelength is a function of both the grating period and the effective refractive index of the device, the output wavelength cannot be accurately controlled from wafer to wafer. Therefore to utilise DFB lasers on a specified frequency, the devices must be sorted by testing and selecting into wavelength ranges and then temperature tuned to attain the desired wavelength. This tuning requirement complicates the temperature stabilisation of the devices, as components such as variable value resistors must be included, making an integrated temperature controller difficult. DBR lasers suffer from similar problems (i.e. changes in effective index and variations in the grating period), with the added difficulty that, due to production tolerances, the relative grating phases of the two Bragg reflectors become unpredictable, resulting in low production yields, or a requirement for a phase adjustment region. In general DBR lasers are more complex and more difficult to manufacture than DFBs.
For use in WDM (wavelength division multiplexing) systems, DFB lasers also require drive-compensating circuits to account for ageing effects which alter the effective refractive index of the device. This also increases the complexity of the control circuit and so may impair long term reliability of the device.
FGLs (fibre grating lasers) are external cavity lasers that essentially consist of a semiconductor laser chip with a reflecting back facet (often HR (high reflection) coated), where lasing is frustrated either by an AR (anti-reflection) coating on the front facet, angled facets/waveguide or a combination of these, coupled to an optical fibre into which a wavelength selective grating is written.
Such gratings are directly written using an UV holographic technique to introduce F-centres (defects) into the fibre in a periodic structure that reflects a selected wavelength. FGLs have the advantage (compared to DFB lasers) that the output wavelength is not determined by the active region and so the wavelength can be accurately pre-determined eliminating the need for sorting and temperature tuning. FGLs also exhibit a smaller wavelength dependence on temperature and so can be operated without temperature compensation over a temperature range of .about.30.degree. C. However, to achieve the same operating range as a DFB laser (0.degree. C.-60.degree. C.), a TEC (thermoelectric cooler) is still required.
The major difficulty with FGLs is in manufacture. The grating write process is essentially serial and therefore unsuitable for volume production. In order to achieve good coupling between the active semiconductor and the fibre, it is usually necessary to use a ball lens or lensed fibre for mode matching. This results in a small alignment tolerance between fibre and semiconductor, hence active alignment is required, another process not suited to volume production.
FGLs are also not suitable for single chip integration with other WDM elements, eg MUX (multiplexers) and DMUX (de-multiplexers).
Silica waveguide grating lasers are similar to FGLs but with the grating written into a silica planar waveguide. These suffer from similar manufacturing problems as FGLs, being a serial write process with the additional problem that the output wavelength can not be accurately pre-selected due to the difficulty in controlling the high temperature doping processes required, leading to a requirement for temperature tuning.